Instruction Examples; Using The Cas And Cas2 Instructions - Motorola MC68030 User Manual

3.5 INSTRUCTION EXAMPLES

The following paragraphs provide examples of how to use selected instructions.

3.5.1 Using the CAS and CAS2 Instructions

The CAS instruction compares the value in a memory location with the value in a data
register, and copies a second data register into the memory location if the compared values
are equal. This provides a means of updating system counters, history information, and
globally shared pointers. The instruction uses an indivisible read-modify-write cycle; after
CAS reads the memory location, no other instruction can change that location before CAS
has written the new value. This provides security in single-processor systems, in
multitasking environments, and in multiprocessor environments. In a single-processor
system, the operation is protected from instructions of an interrupt routine. In a multitasking
environment, no other task can interfere with writing the new value of a system variable. In
a multiprocessor environment, the other processors must wait until the CAS instruction
completes before accessing a global pointer.
The following code fragment shows a routine to maintain a count, in location SYS_CNTR,
of the executions of an operation that may be performed by any process or processor in a
system. The routine obtains the current value of the count in register D0 and stores the new
count value in register D1. The CAS instruction copies the new count into SYS_CNTR if it
is valid. However, if another user has incremented the counter between the time the count
was stored and the read-modify-write cycle of the CAS instruction, the write portion of the
cycle copies the new count in SYS_CNTR into D0, and the routine branches to repeat the
test. The following code sequence guarantees that SYS_CNTR is correctly incremented.
MOVE.W
INC_LOOP MOVE.W
ADDQ.W
CAS.W
BNE
MOTOROLA
SYS_CNTR,D0 get the old value of the counter
D0,D1 make a copy of it
#1,D1 and increment it
D0,D1,SYS_CNTR if countr value is still the same, update it
INC_LOOP if not, try again
MC68030 USER'S MANUAL
Instruction Set Summary
3-25